Assembly and planar structure for use therein which is expandable into a 3-D structure such as a stent and device for making the planar structure

ABSTRACT

An assembly and planar structure for use therein which is expandable into a 3-D structure such as a stent and device for making the planar structure are provided. The planar structure permits the use of planar batch manufacturing technologies to fabricate coronary artery stents. Stents with different wall patterns are fabricated from 50 μm thick stainless steel foil using micro-electro-discharge machining, and expanded to tubular shapes by using angioplasty balloons. The stents are free-standing. The free-standing stents exhibit diameter variations of &lt;±4%, almost zero radial recoil after deflation of the balloon, and longitudinal shrinkage of &lt;3% upon expansion. A variation of the stents uses breakable links to provide additional customization of electrical and mechanical properties. Loading tests reveal that the radial strengths match commercially available stents, while longitudinal compliance, at 0.02 m/N for a 4 mm long section of the stent, is substantially higher.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisionalapplication Serial No. 60/433,846, filed Dec. 16, 2002 and entitled“Design and Fabrication of Stents Using Planar Metal Foils.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under NationalScience Foundation Grant No. ECS-0233174. The Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] This invention relates to assemblies and planar structures foruse therein which are expandable into 3-D structures such as stents anddevices for making the planar structures.

[0005] 2. Background Art

[0006] The following references are noted hereinbelow:

[0007] [1] D. A. Leung et al., “Selection of Stents for Treating IliacArterial Occlusive Disease,” J. VASC. INTERV. RADIOL., Vol. 14, 2003,pp. 137-52.

[0008] [2] P. E. Andersen et al., “Carotid Artery Stenting,” J. INTERV.RADIOL., Vol. 13, No. 3, 1998, pp. 71-6.

[0009] [3] C. R. Rees, “Stents for Atherosclerotic RenovascularDisease,” J. VASC. INTERV. RADIOL., Vol. 10, No. 6, 1999, pp. 689-705.

[0010] [4] C. Pena et al., “Metallic Stents in the Biliary Tree,” MIN.INVAS. THER. & ALLIED TECHNOL., Vol. 8, No. 3, 1999, pp. 191-6.

[0011] [5] B. K. Auge et al., “Ureteral Stents and Their Use inEndourology,” CURR. OPIN. UROL., Vol. 12, No. 3, 2002, pp. 217-22.

[0012] [6] Y. P. Kathuria, “Laser Microprocessing of Stent for MedicalTherapy,” PROC. IEEE MICROMECH. HUMAN SCI., 1998, pp. 111-14.

[0013] [7] K. Takahata et al., “Batch Mode Micro-Electro-DischargeMachining,” IEEE J. MICROELECTROMECH. SYS., Vol. 11, No. 2, 2002, pp.102-10.

[0014] [8] K. Takahata et al., “Coronary Artery Stents Microfabricatedfrom Planar Metal Foil: Design, Fabrication, and Mechanical Testing,”PROC. IEEE MEMS, 2003, pp. 462-5.

[0015] [9] J. C. Conti et al., “The Durability of Silicone Versus LatexMock Arteries,” PROC. ISA BIOMED. SCI. INSTRUM. SYMP., Vol. 37, 2001,pp. 305-12.

[0016] [10] R. C. Hibberler, “Mechanics of Materials Third Edition.”PRENTICE-HALL, INC., 1997.

[0017] [11] S. N. David Chua et al., “Finite-Element Simulation of StentExpansion,” J. MATERIALS PROCESSING TECHNOL., Vol. 120, 2002, pp.335-40.

[0018] [12]. “Metals Handbook Ninth Edition,” Vol. 8 Mechanical Testing,AMERICAN SOCIETY FOR METALS, 1985.

[0019] [13] F. Flueckiger et al., “Strength, Elasticity, and Plasticityof Expandable Metal Stents: In-Vitro Studies with Three Types ofStress,” J. VASC. INTERV. RADIOL., Vol. 5, No. 5, 1994, pp. 745-50.

[0020] [14] R. Rieu et al., “Radial Force of Coronary Stents: AComparative Analysis,” CATHETER. CARDIOVASC. INTERV., Vol. 46, 1999, pp.380-91.

[0021] [15] For example: J. D. Lubahn et al., “Plasticity and Creep ofMetals,” JOHN WILEY & SONS, 1961.

[0022] U.S. Pat. Nos. 6,624,377 and 6,586,699 are related to the presentapplication.

[0023] Stents are mechanical devices that are chronically implanted intoarteries in order to physically expand and scaffold blood vessels thathave been narrowed by plaque accumulation. Although they have found thegreatest use in fighting coronary artery disease, stents are also usedin blood vessels and ducts in other parts of the body. These includeiliac arteries [1], carotid arteries [2], renal arteries [3], biliaryducts [4] and ureters [5]. The vast majority of coronary stents are madeby laser machining of stainless steel tubes [6], creating mesh-likewalls that allow the tube to be expanded radially with a balloon that isinflated during the medical procedure, known as balloon angioplasty.This fabrication approach offers limited throughput and prevents the useof substantial resources available for fabricating planarmicrostructures.

[0024] Micro-electro-discharge machining (μEDM) is another option forcutting metal microstructures. This technique is capable of performing3-D micromachining in any electrical conductor with sub-micron toleranceand surface smoothness. It has not been extensively used for stentproduction in the past because traditional μEDM that uses singleelectrodes with single pulse timing circuits often suffers from evenlower throughput than the laser machining. However, it has been recentlydemonstrated that the throughput of μEDM can be vastly increased byusing spatial and temporal parallelism, i.e., lithographically formedarrays of planar electrodes with simultaneous discharges generated atindividual electrodes [7].

SUMMARY OF THE INVENTION

[0025] An object of the present invention is to provide an assembly andplanar structure for use therein which is expandable into a 3-Dstructure such as a stent and device for making the planar structurewherein the planar structure can be readily manufactured.

[0026] In carrying out the above object and other objects of the presentinvention, a planar structure expandable into a 3-D structure isprovided. The planar structure includes first and second spaced sidebeams which extend along a longitudinal axis. A plurality of spacedcross-bands connect the side beams. together. A first set of thecross-bands are expandable in a first direction substantiallyperpendicular to the longitudinal axis to form a 3-D structure.

[0027] The side beams may be substantially straight and/or continuous.

[0028] A second set of the cross-bands may be expandable in a seconddirection substantially opposite the first direction to form a mesh-like3-D structure.

[0029] Adjacent cross-bands may be expandable in the opposite directionsto form a mesh-like 3-D structure.

[0030] The planar structure may plastically deform during expansion sothat the 3-D structure is free-standing, or may have a cylindricalgeometry.

[0031] The 3-D structure may be a tubular stent.

[0032] The planar structure may include a conductive foil.

[0033] Each of the cross-bands may include a series of folded beams.

[0034] The folded beams may have an involute pattern or a switchbackpattern.

[0035] Each of the cross-bands may include hinges for interconnectingadjacent folded beams.

[0036] The side beams and cross-bands may include biocompatible surfacecoatings.

[0037] The side beams and cross-bands may be made of a biocompatiblemetal.

[0038] The cross-bands may be made of a shape-memory alloy, and theplanar structure may be self-expandable.

[0039] The side beams and cross-bands may be made of at least one of abiocompatible and a biodegradable polymer.

[0040] The side beams and cross-bands may be formed by removing materialfrom a sheet of material.

[0041] The sheet of material may include conductive foil, and the sidebeams and cross-bands may be formed by electric discharge machining theconductive foil.

[0042] At least the first side beam may include a link portion having amechanical strength lower than other portions of the first side beam toallow the first side beam to break at the link portion during expansionof the first set of cross-bands.

[0043] The link portion may be thinned relative to the other portions ofthe first side beam.

[0044] The link portion may be made of a fragile material relative tothe other portions of the first side beam.

[0045] The 3-D structure may be a helical coil.

[0046] The helical coil may include at least one electrical inductor.

[0047] The 3-D structure may comprise at least one electrical conductor.

[0048] The helical coil may include first and second spaced rings atopposite ends thereof. Each of the rings may be formed by an adjacentpair of expanded cross-bands.

[0049] At least the first ring may include a dielectric part whichmechanically connects but electrically insulates adjacent portions ofthe first ring.

[0050] At least the first ring may include a link portion having amechanical strength lower than other portions of the first ring to allowthe first ring to break at the link portion during expansion of thefirst set of cross-bands to open an electrical path formed by the firstring.

[0051] At least one of the side beams and the cross-bands may include adielectric part which mechanically connects but electrically insulatesadjacent portions of the at least one of the side beams and thecross-bands.

[0052] Further in carrying out the above object and other objects of thepresent invention, an assembly including a planar structure is provided.The planar structure includes a pair of spaced side beams which extendalong a longitudinal axis. First and second sets of spaced cross-bandsconnect the side beams together. A balloon is mounted on the cross-bandsso that adjacent cross-bands are disposed on opposite first and secondsides of the balloon. Inflation of the balloon causes the first set ofcross-bands on the first side of the balloon to expand in a firstdirection and the second set of cross-bands on the second side of theballoon to expand in a second direction substantially opposite the firstdirection and substantially perpendicular to the longitudinal axis toform a mesh-like, 3-D structure.

[0053] The balloon may be an angioplasty balloon and the 3-D structuremay be a tubular stent.

[0054] The assembly may further include a catheter tube in fluidcommunication with the angioplasty balloon.

[0055] Still further in carrying out the above object and other objectsof the present invention, a device for use in a electric dischargemachining system to form an expandable planar structure from aconductive planar workpiece is provided. The device includes a substrateand a planar electrode formed on the substrate. The planar electrodeincludes a pair of spaced, side electrode members extending along alongitudinal axis to form a pair of side beams of the structure from theworkpiece. The planar electrode further includes a plurality of spacedcross-band electrode members to form a plurality of spaced cross-bandsof the structure from the workpiece. The cross-bands connect the sidebeams together.

[0056] The side electrode members and the cross-band electrode membersmay include a plurality of copper structures formed by electroplatingthe substrate.

[0057] The substrate may include a semiconductor wafer. The sideelectrode members and the cross-band electrode members may include aplurality of semiconductor structures formed by removing material fromthe semiconductor wafer.

[0058] The above object and other objects, features, and advantages ofthe present invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059]FIGS. 1a-1 c are generalized, schematic views of a planarstructure or stent of the present invention, mounted on a deflatedballoon and expanded to a cylindrical geometry, respectively;

[0060]FIG. 2 is a detailed schematic view of a first embodiment (i.e.,design) of a planar structure of FIG. 3 mounted on a deflatedangioplasty balloon in fluid communication with a catheter tube;

[0061]FIG. 3 is a top plan schematic view of the first embodiment of theplanar structure having a pattern of folded beams in an involute shape;

[0062]FIG. 4 is a top plan schematic view of a second embodiment of theplanar structure having a pattern of folded beams in a switchback shape;

[0063]FIG. 5 is a schematic view of an electric discharge machiningsystem including an electrode for cutting the planar structure of FIG.1a;

[0064]FIG. 6 is a perspective schematic view of the system of FIG. 5with a device including a planar electrode formed on a substrate forcutting a workpiece to form the planar structure of FIG. 1a;

[0065]FIG. 7a is a top plan schematic view of an embodiment of theplanar structure;

[0066]FIG. 7b is a close-up view of a thinned link of a side beam of theplanar structure of FIG. 7a;

[0067]FIG. 8a is a top plan schematic view of another embodiment of theplanar structure;

[0068]FIG. 8b is-a-close-up view of a fragile plug embedded in a sidebeam of the planar structure of FIG. 8a;

[0069]FIGS. 9a-9 d are perspective, schematic views showing a planarstructure having breakable links by itself, on a deflated balloon, on afully expanded balloon, and its electrical path equivalent to the finalstage of FIG. 9c, respectively, wherein the resulting 3-D structureincludes a helical coil with end rings;

[0070]FIG. 10a is a top plan schematic view of another planar structurehaving breakable links;

[0071]FIG. 10b is a simplified view of a 3-D structure in the form of apair of helical inductors formed after the expansion of the planarstructure of FIG. 10a;

[0072]FIG. 11a is a perspective, simplified view of an expanded 3-Dstructure having end rings;

[0073]FIG. 11b is a close-up view of plugs of a dielectric materialembedded in the end rings of FIG. 11a;

[0074]FIG. 12a is a perspective, simplified view of an expanded 3-Dstructure having end rings;

[0075]FIG. 12b is a close-up view of breakable links or portions of theend rings of FIG. 12a;

[0076]FIG. 13a is a top plan schematic view of another embodiment of aplanar structure;

[0077]FIG. 13b is a close-up view of an interconnecting dielectric plugused in the planar structure of FIG. 13a; and

[0078]FIG. 14 is a perspective, schematic view of folded beams having aninvolute shape in initial (indicated by dashed lines) and expanded(indicated by solid lines) positions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0079] A new assembly and planar structure for use therein which isexpandable into a 3-D structure such as a stent and device for makingthe planar structure are disclosed herein. Also, this invention presentsa new approach to the design and manufacture of coronary artery stents,which permits the use of planar batch fabrication techniques usingmicroelectrodischarge machining. The devices are compatible withstandard stenting tools and procedures. The wall patterns were designedso that both stress relief and the mechanical strength aresimultaneously achieved in the expansion.

[0080] Referring to FIGS. 1a-1 c, a generalized schematic view of astent or planar structure constructed in accordance with the presentinvention is generally indicated at 10. The stent 10 is mounted on adeflated balloon 12 in FIG. 1b and on the balloon 12 when inflated inFIG. 1c. The stent 10 includes involute bands 16 tied between a pair ofside beams 14. Measurements demonstrate that the designs have the sameradial strength as a commercial stent even though the former use metalthat is only about half as thick. The thinner walls also contributed toachieving higher longitudinal flexibility than a commercial one in theexpanded state. Both the radial strength and the flexibility are foundto have no significant dependence on orientation relative to theoriginal planar direction of the foil. Dimensional variations in tubulardiameter, longitudinal shrinkage, and radial recoiling in the expandedstents are at most a few percent.

[0081] The invention will also facilitate other three-dimensionalstructures such as antennas and transformers. Using this approach, anyelectrically conductive material can be used to form a tubular mesh-likestructure. This includes structures which have attached elements that donot conform to the shape of the cross-section of the tube, such astangential cantilever or loop attached to the perimeter. The structurescan be used as inductors (i.e., FIG. 10b), antennas, transformers, orcapacitors for electrical circuits. They may also be used for mechanicalfunctions such as springs, trusses, etc. in microsystems.

[0082] The new fabrication approach uses metal foils as startingmaterials for the stents 10, which permits the parallelism described inU.S. Pat. Nos. 6,624,377 and 6,586,699 to be exploited, thereby offeringhigh throughput and repeatability. The favored mechanicalcharacteristics including radial strength and longitudinal compliance inexpanded stents (i.e., FIG. 1c) have been experimentally andtheoretically investigated, and are discussed with comparisons tocommercial stents.

[0083] A variation that uses strategically located breakable links(FIGS. 7a, 8 a, 9 a, 10 a) in the stent provides additional freedom incustomizing the mechanical and electrical properties of these devices.

[0084] Design and Fabrication

[0085] The fabrication approach was applied to μEDM 50 μm-thickstainless steel foil into a planar structure, generally indicated at 20,that could be slipped over an angioplasty balloon 22 and be reshapedinto a cylinder when deployed in the manner of a conventional stent viaa catheter tube 28, as shown in FIG. 2. The planar pattern of thestructure 20 provides the important mechanical characteristics of radialstiffness and longitudinal compliance in the expanded structure. Inorder to reduce the likelihood of joint failure, it was decided todevelop a structure 20 that was completely flexural in nature, and didnot have any bonded or hinged joints. This effort used 50 μm-thick type304 steel which is very similar to the 316 steel commonly used forcommercially available stents.

[0086] Several layouts were designed and experimentally tested. The bestresults in terms of mechanical characteristics (discussed herein below)were obtained with the design shown in FIGS. 2 and 3, which is referredto as design 1 (i.e., units are in μm). The pattern has two longitudinalside beams 24, which are connected transversely by cross-bands 26, eachof which contain three identical involute loops (i.e., FIG. 14 shows onesuch loop). The involute shape is tailored to provide selectedstress-relief during expansion of the stent 20 to the desired deploymentdiameter, which is 2.65 mm in this case. In order to increase radialstrength, this design uses a larger number of cross-bands 26 per unitlength of the stent 20 and beams A_(n), C_(n), and E_(n) are designed tobe longer than the others, B_(n) and D_(n).

[0087] Another representative planar substrate is illustrated at 40 inFIG. 4 (i.e., units again in μm), which is referred to as design 2. Ithas similar dimensions and a configuration that uses an array ofcross-bands 42 and two side beams 44, but the cross-bands 42 have aswitchback pattern in this case. In contrast to the involute design, thebeams in segment G, which are parallel to the longitudinal axis, arelonger than the others in segment H, which are perpendicular to theaxis. This design, in general, has a higher expansion ratio to theinitial width between the side beams 44 in radial direction, but fewercross-bands 42 along the longitudinal axis.

[0088] To emulate the deployment of a stent, the angioplasty balloon 22was threaded through the 7 mm-long planar structure 20, as shown in FIG.2, such that the transverse bands 26 alternated above and below it. Withthe set-up illustrated in FIGS. 1b and 2, the stent 20 was expanded byinflating the balloon 22 with liquid up to 12 atm. pressure, in a manneridentical to commercial stents, resulting in the structure similar tothe one shown in FIG. 1c. Variation in the diameter of expanded stentswas typically within ±4%, while radial recoil upon deflation of theballoon 22 was even smaller than that. The shrinkage in length upon theexpansion was <3%. A deployment inside a mock artery was done. The mockartery used was a commercially available silicone-based tube (DynatekDalta Scientific Instruments, MO, USA) with 3 mm diameter and 0.25 mmwall thickness, which is tailored to have radial compliance comparableto human coronary arteries [9]. In this deployment, the stent 20 wasexpanded to 3.5 mm diameter. The tube had a distended sidewall at thelocation where the stent 20 was deployed, demonstrating mechanicalstrength large enough to prevent the relaxation of the simulated artery.

[0089] Upon expansion of the stent, beams in the structure arepermanently deformed as shown in FIG. 14. The pattern of the stent must,therefore, be designed to accommodate large deformations so that themaximum tensile stress is less than the ultimate stress, which is about517 MPA for the 304 stainless steel [10]. The deformation and resultantstresses were evaluated by using an FEA package, ANSYS™. The simulationused a bilinear stress-strain model, and the following mechanicalproperties of the steel [10,11]: Young's modulus=193 GPA, yieldstress=207 MPA, tangent modulus=692 MPA, and Poisson's ratio=0.27. FIG.14 shows a unit involute section, generally indicated at 140, of thecross-bands of design 1 with a displacement that approximatelycorresponds to the deployed diameter. The section 140 includes beams 142with interconnecting hinges 144. The maximum von Mises stress appears atthe location indicated near the flexural hinge element B_(n) and is 382MPA, sufficiently below the ultimate stress.

[0090] In addition to the bending of beam segments, torsionaldeformations also play important roles in expanding a stent andmaintaining its final shape. The most significant ones are in the sidebeams 24, which are twisted by 90-180° along the segment F (labeled inFIG. 3) between two adjacent bands 26. Different torsional deformationwas observed at a flexural hinge H in design 2 (i.e., FIG. 4). Theapproximate shear strain for both these cases was shown on a shearstress versus shear strain response curve for 304L stainless steelobtained from [12]. It was evident that beam fracture associated withonly the torsion is not a concern for the stent. For the test, the hingeas well as the beams had 50 μm square cross-section. Although the straindue to this torsion is well below the fracture point, additionaldeformations at the site also include bending that may further increasethe maximum strain experienced. Mechanical failure was observed due to acombination of severe bending and tension. This fracture was observed indesign 1 _(A), a precursor to design 1 for which width of flexuralhinges was 50 μm, and segments A_(n), (and E_(n)), B_(n) (and D_(n)),and C_(n) were 550, 150, and 450 μm, respectively. The narrower widthand shorter length in the flexural hinges, B_(n) and D_(n) of thisdesign contributed increasing the tensile stress at the hinge. Sincethis was the only failure experienced, it is likely that an instance ofmetallurgical defect may have contributed to it. In design 1, a largersafety margin was incorporated by two changes: (i) doubling the widthsof the segments B_(n) and D_(n) from 50 μm to 100 μm, and (ii)increasing the lengths of the same segments from 150 μm to 200 μm bydoubling the gap between adjacent beams, as seen in FIG. 3.

[0091] In addition to the stent fabrication, the planar scheme can beeasily extended to fabrication of 3-D inductors, generally indicated at108 in FIG. 10b. The final 3-D structure is essentially a set ofseries-connected rings which offer negligible inductance. Use ofbreakable links in planar structures 70, 80, 90 and 100 (FIGS. 7a, 8 a,9 a, and 10 a) permits formation of helical coils or inductors 98 and108 (FIGS. 9d with electrical connection 99 and 10 b, respectively) inthe same manner for the deployment of stents. FIGS. 7a and 7 b showthinned links 78. FIG. 9a shows thinned breakable links 94 in the planarstructure 90 including cross-bands 96. FIGS. 8a and 8 b show fragileplugs 88 to make its links breakable.

[0092] When a balloon 92 is inflated for expanding the planar structure90 (FIG. 9c), torsional strain developed in the side beams 91 iseffectively concentrated at the links 94 made in the beams 91 (FIG. 9a),leading to fracture (FIG. 9c). The resultant final shape can be helicalby placing the links 94 at selected locations. This fracture iscontrolled breakage, and the fractured cross-section area is minimal.

[0093] In like fashion, torsional strain developed in side beams 101(i.e., FIG. 10a) is effectively concentrated in breakable links 104 inthe beams 101 leading to fracture as shown in FIG. 10b with electricalconnection 109.

[0094] Experimental Results

[0095] The radial strength is a paramount mechanical characteristic inthe stents. Several past efforts have assessed the strength incommercial stents [13,14]. To evaluate the devices of the presentinvention, short samples for involute and switchback designs wereprepared and subjected to loading tests in which the reaction force perunit length of the stent is measured as a function of radial deformation. A sample is held in a groove mounted on the stage and compressedtoward the probe. The gauge is rigidly fixed, and the displacement ofthe gauge probe is negligible compared to that of the sample. The forcewas measured by a gauge (Imada, Inc., IL, USA, DPS-1) that provides 1 mNresolution while first compressing the stent by 1.5 mm in 25 μmincrements, and then while relaxing the deformation.

[0096] A commercial stent with 316 stainless steel of thickness varyingover 90-130 μm was tested for comparison. Measurements demonstrate thatthe design that uses the involute cross-bands (design 1 _(A)) has thesame radial strength to the commercial stent with similar diameter andtwice the thickness. In addition, it exhibits better elastic recoveryafter loading, which suggested that it has better radial elasticity butthe same stiffness as the commercial one. The switchback pattern (design2), which as fewer cross-bands per unit longitudinal length, providesless radial strength than the involute pattern.

[0097] Orientation dependence of the radial strength was a concern sincethey were shaped from planar sheets as shown in FIG. 6. Identicalsamples with four cross-bands of design 2 were tested at two differentorientations: (A) perpendicular to a plane that includes both sidebeams, and (B) parallel to the plane. The measurements demonstrate thatthe radial strength is similar in both cases.

[0098] The experimental results showed a few discontinuities in theresponse curve. As can be seen in FIG. 14, beams that correspond to {dotover (C)}_(n) in FIG. 3 are designed to rotate about their center by−90° during the expansion. As a result, hinges D_(n), and B_(n+2) arepositioned closely to each other. In addition, alternate cross-bands 26in FIG. 3, which adjoin each other when they are mounted on the balloon22, deform in a way that the gaps between their segments are reduced asthe stent 20 expands since the side beams 24 are deformed to wave-likeshapes. The combination of these effects results in increasedprobability of physical contact between the hinges D_(n) and B_(n+2) asthe balloon 22 is being inflated. As loading is applied, hinges happento come into contact and get intermeshed, and then snap apart as theloading is further increased. This particular sample, being design 1_(A), had a reduced gap of 50 μm between the cross-bands 26, which couldalso contribute to increase the probability. This undesirable mechanicalinteraction however can be improved by optimizing the layout.

[0099] Longitudinal compliance is a favored characteristic in stents.This is because the stent, fitted on an angioplasty balloon in a statethat is only slightly expanded, must often travel a convoluted pathalong a blood vessel in order to reach the location of the deployment.In addition, longitudinal flexibility in a fully expanded stent can bebeneficial for its deployment at curved sites. The longitudinalcompliance of the fabricated stents was tested. A fully-expanded 7 mmlong stent of design 1 was attached to a holder such that a 4 mm segmentout of it was overhanging and unsupported. Using a force gauge, thedisplacement response was plotted for an end load. A similar test wasalso applied to the commercial stent tested before. The results revealthat the stent of the present invention had spring constants of 50 N/mand <5 N/m depending on the orientation, whereas that in the commercialstent resulted in 515 N/m. While this test was only performed onexpanded stents, it suggests that the stents of the present inventionperform favorably in this respect.

[0100] The design and fabrication of coronary artery stents of thepresent invention is preferably based on use of planar stainless steelfoil and μEDM technology, as generally shown in U.S. Pat. No. 6,624,377.An electrode 52 is controlled by a control unit in FIG. 5. FIG. 6 showsa workpiece 64 processed by a device including a substrate 66 on whichis formed a planar electrode, generally indicated at 68, having side andcross-band electrode members 60 and 62, respectively.

[0101] The devices are intended to be compatible with standard stentingtools and procedures. The wall patterns were designed using FEA so thatboth the stress relief and the mechanical strength are simultaneouslyachieved in the expansion. The devices include involute bands tiedbetween a pair of side beams. Measurements demonstrate that the designshave the same radial strength as a commercial stent even though theformer use metal that is only about half as thick. The thinner wallsalso contributed to achieving at least 10× higher longitudinalflexibility than a commercial one in the expanded state. Both the radialstrength and the flexibility are found to have no significant dependenceon orientation relative to the original planar direction of the foil.Dimensional variations in tubular diameter, longitudinal shrinkage, andradial recoiling in the expanded stents are at most a few percent.

[0102] All devices tested in this effort were fabricated bybatch-compatible μEDM, which can open a path to exploitphotolithography-based fabrication resources for the stent production[7]. As an extension of this technology for manufacturing stents, use ofstrategically-located breakable links as described above also facilitatefabrication of other 3-D structures such as antennas and transformers.

[0103] Furthermore, referring to FIGS. 13a and 13 b, dielectric plugs132 may be incorporated into a planar structure 130 to ensure thatelectric current does not attempt to flow in end rings in the expanded3-D structure (not shown).

[0104]FIGS. 11a and 11 b show an expanded 3-D structure in the form of ahelical coil 110 having end rings 112 with such dielectric embeddedplugs 114.

[0105]FIGS. 12a and 12 b also show an expanded 3-D structure in the formof a helical coil 120 having end rings 122 with thinned portions 124 sothat the end rings 122 are breakable to prevent current flow in the endrings 122.

[0106] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A planar structure expandable into a 3-Dstructure, the planar structure comprising: first and second spaced sidebeans which extend along a longitudinal axis; and a plurality of spacedcross-bands which connect the side beams together wherein a first set ofthe cross-bands are expandable in a first direction substantiallyperpendicular to the longitudinal axis to form a 3-D structure.
 2. Theplanar structure as claimed in claim 1, wherein a second set of thecross-bands are expandable in a second direction substantially oppositethe first-direction to form a mesh-like 3-D structure.
 3. The planarstructure as claimed in claim 2, wherein adjacent cross-bands areexpandable in the opposite directions to form a mesh-like 3-D structure.4. The planar structure as claimed in claim 1, wherein the planarstructure plastically deforms during expansion so that the 3-D structureis free standing.
 5. The planar structure as claimed in claim 2, whereinthe planar structure plastically deforms during expansion so that the3-D structure has a cylindrical geometry.
 6. The planar structure asclaimed in claim 2, wherein the 3-D structure is a tubular stent.
 7. Theplanar structure as claimed in claim 1, wherein the planar structureincludes a conductive foil.
 8. The planar structure as claimed in claim1, wherein each of the cross-bands includes a series of folded beams. 9.The planar structure as claimed in claim 8, wherein the folded beamshave an involute pattern.
 10. The planar structure as claimed in claim8, wherein the folded beams have a switchback pattern.
 11. The planarstructure as claimed in claim 8, wherein each of the cross-bandsincludes hinges for interconnecting adjacent folded beams.
 12. Theplanar structure as claimed in claim 1, wherein the side beams andcross-bands include biocompatible surface coatings.
 13. The planarstructure as claimed in claim 1, wherein the side beams and cross-bandsare made of a biocompatible metal.
 14. The planar structure as claimedin claim 1, wherein the cross-bands are made of a shape-memory alloy andwherein the planar structure is self-expandable.
 15. The planarstructure as claimed in claim 1, wherein the side beams and cross-bandsare made of at least one of a biocompatible and a biodegradable polymer.16. The planar structure as claimed in claim 1, wherein the side beamsand cross-bands are formed by removing material from a sheet ofmaterial.
 17. The planar structure as claimed in claim 16, wherein thesheet of material includes conductive foil and wherein side beams andcross-bands are formed by electric discharge machining the conductivefoil.
 18. The planar structure as claimed in claim 1, wherein at leastthe first side beam includes a link portion having a mechanical strengthlower than other portions of the first side beam to allow the first sidebeam to break at the link portion during expansion of the first set ofcross-bands.
 19. The planar structure as claimed in claim 18, whereinthe link portion is thinned relative to the other portions of the firstside beam.
 20. The planar structure as claimed in claim 18, wherein thelink portion is made of a fragile material relative to the otherportions of the first side beam.
 21. The planar structure as claimed inclaim 18, wherein the 3-D structure is a helical coil.
 22. The planarstructure as claimed in claim 21, wherein the helical coil comprises atleast one electrical inductor.
 23. The planar structure as claimed inclaim 21, wherein the helical coil includes first and second spacedrings at opposite ends thereof and wherein each of the rings is formedby an adjacent pair of expanded cross-bands.
 24. The planar structure asclaimed in claim 23, wherein at least the first ring includes adielectric part which mechanically connects but electrically insulatesadjacent portions of the first ring.
 25. The planar structure as claimedin claim 23, wherein at least the first ring includes a link portionhaving a mechanical strength lower than other portions of the first ringto allow the first ring to break at the link portion during expansion ofthe first set of cross-bands to open an electrical path formed by thefirst ring.
 26. The planar structure as claimed in claim 1, wherein atleast one of the side beams and the cross-bands includes a dielectricpart which mechanically connects but electrically insulates adjacentportions of the at least one of the side beams and the cross-bands. 27.An assembly comprising: a planar structure including: a pair of spacedside beams which extend along a longitudinal axis; and first and secondsets of spaced cross-bands that connect the side beams together; and aballoon mounted on the cross-bands so that adjacent cross-bands aredisposed on opposite first and second sides of the balloon whereininflation of the balloon causes the first set of cross-bands on thefirst side of the balloon to expand in a first direction and the secondset of cross-bands on the second side of the balloon to expand in asecond direction substantially opposite the first direction andsubstantially perpendicular to the longitudinal axis to form amesh-like, 3-D structure.
 28. The assembly as claimed in claim 27,wherein the balloon is an angioplasty balloon and the 3-D structure is atubular stent.
 29. The assembly as claimed in claim 28, furthercomprising a catheter tube in fluid communication with the angioplastyballoon.
 30. A device for use in a electric discharge machining systemto form an expandable planar structure from a conductive planarworkpiece, the device comprising: a substrate; and a planar electrodeformed on the substrate and including a pair of spaced, side electrodemembers extending along a longitudinal axis to form a pair of side beamsof the structure from the workpiece and a plurality of spaced cross-bandelectrode members to form a plurality of spaced cross-bands of thestructure from the workpiece, the cross-bands connecting the side beamstogether.
 31. The device as claimed in claim 30, wherein the sideelectrode members and the cross-band electrode members comprise aplurality of copper structures formed by electroplating the substrate.32. The device as claimed in claim 30, wherein the substrate includes asemiconductor wafer and wherein the side electrode members and thecross-band electrode members comprise a plurality of semiconductorstructures formed by removing material from the semiconductor wafer. 33.The planar structure as claimed in claim 1, wherein the side beams aresubstantially straight and continuous.
 34. The planar structure asclaimed in claim 1, wherein the side beams are substantially straight orcontinuous.
 35. The planar structure as claimed in claim 1, wherein the3-D structure comprises at least one electrical conductor.